Sediment entrainment from patches of fine material in a gravel-bed river: Measurements and Experiments

Abstract

Patch entrainment is a relatively frequent geomorphic process that holds important implications for stream ecology. This paper analyses bedload entrainment from patches of fine sediment in a gravel-bed river from field experiments using a portable flume. Hiding exerts a marked effect on entrainment, multiplying by four the critical shear stress for initiation of motion predicted by the Shields function. Once particles are set in motion, patches of sand and fine gravel act as the main source of sediment for bedload, although rates remain very low (<6 g/sm). Such values fit within the category of marginal transport described elsewhere in the literature and correspond with typical hydraulic conditions attained during frequent low magnitude flood events (e.g. <10 N/m2). Comparison between experimental results and data obtained by means of pressure-difference samplers at the same river reach indicates that the threshold between marginal and selective transport occur at around 30 N/m2. This value is associated to ~20% bankfull discharges that in this river are equalled or exceeded around 15% of the time.